Gridded electron tube with dispenser cathode having coated surface portions



Sept 17,1968 M. T. VLAARDINGERBROEK ETAL 3,402,314

. GRIUUED ELECTRON TUBE WITH DISPENSER CATHODE HAVING COATED ,SURFACEPORTIONS Filed March 15, 1966 FIGJ FIG.3

MARINUS 'LVLAAMXEMK EDUARD GDORGELO BY AGENT United States Patent OABSTRACT OF THE DISCLOSURE An electron tube having a grid with pluralapertures` opposite a dispenser cathode. The cathode surface portionsonly opposite the grid apertures being coated with Rh, Ir, Ru, or Os toincrease their emission relative to the uncoated portions, causing theelectrons to be emitted as beams aligned with the grid apertures. Thisreduces heating of the grid as well as offering other improvements.

The invention relates to an electric discharge tube cornprising acathode having a metallic emissive surface opposite one or more grids,and in particular to a high power tube for use at frequencies in excessof 3000 mc./s.

Tubes operating at such high frequencies entail great diiiiculties dueto excessive heating of the first grid. In order to avoid excessiveheating of this first grid, it is known to construct it as a solid discwith apertures across which the grid wires are stretched. In order tominimize grid current, it is also known to shape the cathode, an oxidetype, in a form such that only the portions of the cathode surfacelocated opposite the apertures are coated with the oxide emissivematerial. Such constructions involve the difficulty that the barium ofthe active cathode portions will migrate over the cathode surface at itsoperating temperature and ultimately cover also the nonemissiveportions, which are thereby rendered emissive as well.

Since such an oxide emissive layer has further disadvantages at veryhigh frequencies, it is preferred to use a so-called dispenser typecathode with a metallic emissive surface. However, the operatingtemperature of such a dispenser cathode is much higher than that ofbarium oxide cathodes, e.g., 1050 C., as a result of which thedifficulties in connection with the excessively high grid temperatureare aggravated, since a dispenser cathode causes increased heating ofthe grid by radiation, and, moreover, the electron flow is` alsoincreased. While it might inhibit emission from those portions of theemissive surface located opposite the closed portions of the grid bycoating with a carbide or zirconium, the disadvantage of the higheroperating 'temperature of the dispenser cathode remains and theconcommitant difflculties due to the strong heat radiation to the Igrid.Moreover, the filament wire insulation is more severely taxed due to thehigher operating temperature.

In a prior copending application, Ser. No. 410,655, filed Nov. l2, 1964,now U.S. Patent No. 3,373,307, whose contents are hereby incorporated byreference, has been described an improved dispenser cathode containingon its emissive surface a thin porous coating of osmium, iridium,ruthenium or rhenium, which Offers the advantage of affording the sameemission density but at a substantially lower temperature.

In accordance with the present invention, the difficulties encounteredin the construction of a high power, high frequency electron tube, aspreviously described. are sub- 3,402,314 Patented Sept. 17, 1968 ICCstantially completely avoided, by incorporating in the tube a dispensercathode whose active portions located opposite the grid apertures arecoated with a thin coating of metal of the group of rhenium, iridium,ruthenium or osmium as described in the copending application. As aresult, the cathode can be operated at a temperature at least C. lowerthan the conventional value of 1050 C. for the ordinary dispensercathode. Owing to the lower operating temperature, the emission of thenon-coated portions of the cathode surface is strongly reduced. As afurther improvement, the non-coated cathode surface portions may becovered with a substance inhibiting electron emissivity, for example, acarbide as described' in U.S. Patent No. 2,972,078, or zirconium asdescribed in U.S. Patent No. 2,895,070. The lower cathode temperaturehas the advantage of reducing the heating energy required for thecathode and also reducing heating of the grid by cathode radiation; alsothe insulation of the filament wire is improved. Moreover, the lifetimeof the cathode is considerably prolonged.

The invention will now be described more fully with reference to theaccompanying drawing, in which:

FIG. 1 is a cross-sectional View of one form of a discharge tubeaccording to the invention;

FIG. 2 is a plan view of the grid of such a tube, and

FIG. 3 is a plan View of the emissive surface of the cathode used in thetube of FIG. 1.

The discharge tube shown in FIG. 1 is a so-called discseal triode forvery high frequencies and high power. It comprises a dispenser cathode1, a grid electrode 2 and an anode 3 separated -by the usual insulatorsforming the tube enevelope. The grid electrode 2, in the form of aterminal disc, has secured to it a metal plate or disc 4 havingelongated apertures or slots 5, across which grid wires 6 are stretched(FIG. 2). These wires 6 are soldered or welded to the plate 4 so that asatisfactory thermal contact between the wires 6 and the plate 4 isestablished and the wires 6 are, consequently, cooled satisfactorily.

`The dispenser cathode 1 comprises a porous tungsten body 7, and isconstructed as described in the copending application as a cavity-typeor impregnated type, with the exception that, instead of the wholeemissive surface 9 of the cathode being coated with the high workfunction metal Rh, Ir, Ru, Os, only the surface areas 8 (FIG. 3) locatedopposite the slots 5 of the grid plate 4 are coated with the rhenium,irridium, ruthenium or osmium, The work function of the portions 8 ofthe cathode surface is thus reduced with respect to that of the initialtungsten surface 9 to an extent such that the Iportions 8 have asatisfactory emission already at a cathode temperature more than 100 C.lower than the conventional value of 1050o C., at which the uncoatedcathode surface portions 9 then emit only a small amount of electrons.The electron flow thus travels in the form of beams just through theapertures 5 of the grid plate 4. Owing to the improved cooling of thegrid and to the lower temperature of the grid due to cathode radiationas a result of the lower cathode temperature, the advantages of adispenser cathode with a metallic, emissive surface in a tube for veryhigh frequencies can thus be utilized to a considerably higher extentwithout incurring the risk of excessive heating of the grid.

If the emission of the portions 9 of the cathode surface has to besuppressed more completely, these portions may be covered with aninhibitor such as zirconium, which offer the additional advantage thatthis surface also possesses gas-binding properties. It is found with theuse of osmium, rhenium, ruthenium or iridium, that the separationbetween the portions of good emission and those of poor emission of thecathode surface is maintained during operation, since the osmium,rhenium, ruthenium or iridium do not migrate but remain where deposited,and

are not evaporated like barium, while the barium supplied from thecathode interior and which covers both the portions 8 and the portions 9of the cathode surface is capable of providing a satisfactory electronemission only in cooperation with the osmium, rhenium, ruthenium oriridium coated portions 8. In a typical impregnated dispenser cathode,the member 7 would be of porous tungsten with a density of the order of80% with the pores filled by impregnation from the melt withbarium-calcium aluminate, as described in the copending application. Thehigh work function coating is provided, also as described in thecopending application, by cathode sputtering or spraying through asuitable mask to form the stripe pattern illustrated in FIG. 3.

Although a single embodiment is described above, the invention is alsoapplicable to tubes containing other types of cathodes having a metallicemissive surface.

While we have described our invention in connection with specificembodiments' and applications, other modifications thereof will bereadily apparent to those skilled in this art without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. An electron tube comprising cathode, grid and anode electrodes, saidgrid having plural apertures located between the cathode and the anodefor allowing electrons to fiow from the cathode to the anode, saidcathode `being a dispenser type cathode having a metallic surface withportions located opposite the grid apertures separated by portionslocated opposite solid parts of the grid, and a coating of a metalselected from the group consisting of rhenium, iridium, ruthenium andosmium covering only those portions of the cathode surface opposite thegrid apertures, whereby said coated portions exhibit an adequateelectron emission at a cathode temperature lower than the temperature towhich the uncoated portions would have to be raised to generate adequateelectron emission.

2. A tube as set forth in claim 1 wherein the cathode is operated at atemperature at least C. lower than 1050 C.

3. An electron tube as set forth in claim 1 wherein the grid comprises asolid disc containing spaced apertures with plural grid wires secured tothe disc across the apertures.

4. A tube as set forth in claim 3 wherein the apertures are in the formof parallel slots extending in directions transverse to the grid wires,and the coated cathode surface portions are in the form of stripessubstantially matching the grid aperture configurations.

5. An electron tube as set forth in claim 1 whein the non-coated cathodesurface portions are covered with an emission inhibitor selected fromthe group consisting of carbides and zirconium.

6. A tube as set forth in claim 1 wherein the cathode comprises a poroustungsten stubstrate for the coating and is associated with abarium-calcium-aluminate activator.

No references cited.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

